X-ray intensifying screens have in the past been used in combination with silver halide containing radiographic films to reduce the exposure of patients to X-rays. Intensifying screens typically consist of a support and a phosphor layer. The phosphor layer, which absorbs X-radiation more efficiently than the silver halide, is capable of emitting a longer wavelength radiation to the adjacent silver halide emulsion layer of the radiographic element in an image pattern corresponding to that of the X-radiation received.
The efficiency of absorption of the X-radiation by the phosphor is attributable to some extent to the density of the phosphor crystals. Higher density phosphors absorb radiation more efficiently than lighter compositions. One common phosphor composition, gadolinium oxy sulfide (Gd.sub.2 O.sub.2 S), has a density of about 7.3 g/cm.sup.3. It would be desirable to develop a variety of useful phosphor host compounds having higher densities, since theoretically, for such phosphors thinner intensifying screens could be employed, resulting in increased resolution.
Several such high density luminescent compositions have been decribed. Titanium activated hafnium oxide, for example, has a density of about 9.7 g/cm.sup.3. Phosphor compositions of this type are described in U.S. Pat. No. 4,988,880 to Bryan et al. Other high density phosphor compositions include hafnium germanate and lutetium tantalate.
In addition, there are a variety of intensifying screens, each of which emit different wavelengths of the electromagnetic spectrum. Consequently, there is a continuing need for high density phosphor compositions which emit at different wavelengths. There is also a continuing need for phosphors which exhibit improved luminescence intensities.